1. Field of the Invention
The present invention provides a light emitting device and a preparation method of the same. In particular, the present invention provides a white-light emitting device and a preparation method of the same.
2. Descriptions of the Related Art
White light is a mixed light comprised of many colors. The light sensed by a human eye as white light comprises a mixed light of at least two wavelengths. For example, when a human eye is exposed to red, blue and green lights at the same time, or perceives blue and yellow lights at the same time, the eye perceives a white light. Thus, a white light emitting diode (LED) can be made according to this principle.
White light LEDs have been one of the most important and valued products in the world. Compared to conventional incandescent light bulbs, white light LEDs have the advantages of such as small in volume, emit no thermal radiation, consume low power, long service life and good reaction speed, and provide resolutions for many problems which were difficult for the incandescent light bulb to overcome in the past.
Accordingly, developed countries such as Europe, America, and Japan use white light LEDs as a new light source in the 21st century based on the common consensus of saving energy and protecting the environment. In Taiwan, about 11 billion kilowatts-hour of the power can be saved per year if a quarter of incandescent light bulbs and all fluorescent lamps are replaced by white light LEDs, which almost equals the yearly power generation of a nuclear power station. In view of this, it is clear that the benefit obtained from the development of white light LEDs is incredible. Therefore, developed countries such as Europe, America, and Japan have been invested a lot of manpower in the development of white light LEDs. It is expected that conventional lighting apparatuses will be replaced by white light LEDs in the following ten years.
The preparation method for white light LEDs was first developed by the Nichia Corporation, which mixed light of two wavelengths. In this method, a layer of yttrium aluminum garnet phosphor was coated on a blue light LED having a primary light emitting wavelength of 460 nm, and then the yttrium aluminum garnet phosphor was excited by the blue light LED to generate a yellow light having a primary wavelength of 555 nm, which is a complementary light of the blue light. The complementary lights, i.e., the yellow and the blue lights were mixed through the lens theory to obtain the desired white light.
Many people in the lighting industry are invested in developing a tri-wavelength light because the patent of the aforesaid preparing method is held by Nichia Corporation. The three primary color phosphors of blue, green, and red lights are excited by the UV light provided by an inorganic UV LED chip to generate the tri-wavelength light. If the ratio of the generated three primary color lights is exact, then the mixed light thereof will be a white light. However, to excite the various phosphors to generate fluorescence at the same time, one prerequisite condition is that the selected exciting light source must be absorbed by the fluorescence; the difference of the absorption coefficient for the exciting light between the phosphors cannot be too much; and the quantum efficiency of the photon conversion of the phosphors should also be as similar as possible. Obviously, the species of proper fluorescence materials are very limited. In addition to the difficult material selection, the color mixing equation for using three (or more) phosphors is a non-linear equation of second-order or more, based on the color mixing principle. In other words, its color change rate is two-dimensional or more than two-dimensional, and therefore, adjusting the ratio of the three primary color phosphors to obtain the white light is more difficult in technique.
In view of this, the present invention provides a white-light emitting device and a method for preparing the same, wherein only two species of phosphors are used to provide the desired white light.